There are numerous patents and published applications in the multistage hydroprocessing area which disclose one or more of the following features: product recycle, interstage separation and interstage feed addition. U.S. Pat. No. 6,200,462 discloses recycle of a portion of bottoms material (bottoms for first and second stages were combined and fractionated) to the first stage, where it is combined with fresh feed prior to entering the first stage. Interstage separation occurs after the first stage. The vapor stream is passed to a second stage hydrprocessing unit, and the liquid stream is fractionated.
U.S. Pat. No. 6,787,025 discloses two stage hydroprocessing with interstage separation in a hot high pressure separator. The vapor stream is subjected to further processing and the bottoms from the hot high pressure separator proceeds to fractionation. External feed is added to the vapor stream prior to further processing.
U.S. Pat. No. 6,797,154 discloses two stage hydroprocessing with interstage separation in a hot high pressure separator. External feed may be added to the vapor stream as it leaves the separator. The vapor stream undergoes fractionation and optional further hydroprocessing. The liquid stream is processed in a second hydroprocessing unit and the effluent sent to a cold high pressure separator. Heavier materials from the separator are sent to fractionation and the lighter materials are recycled to the first stage.
U.S. Pat. No. 6,623,624 discloses two stage hydroprocessing with interstage separation. Effluent from the first hydroprocessing unit passes to atmospheric separation where a first fuel product is removed, and the heavy fraction then proceeds to a vacuum separation zone where fuel and lubricant products are removed. The bottom fraction of the vacuum separation zone the proceeds to a hydrocracking zone, where additional fuel and lubricant products are removed.
In one conventional two stage hydroprocessing scenario, a fresh feed is reacted in a first hydrocracking stage. Effluent from the first stage is combined with effluent from a second stage and the blend fractionated. Distillate fuel product is recovered, and the bottoms product from the fractionator passed to a second hydrocracking stage for further conversion.
It is important to differentiate the “bleed” of a conventional two-stage hydrocracker from the deeply hydrogenated heavy product which is produced in this process. This differentiation is further discussed in the Detailed Description of the Invention.